1. Field of the Invention
The present invention relates to a semiconductor device, such as a thin film transistor, which is appropriately used in a liquid crystal display, an image sensor, a solar cell, etc. The present invention also relates to a method for producing the same and to a method for producing a liquid crystal display apparatus utilizing such a semiconductor device.
2. Description of the Related Art
Recently, when producing a liquid crystal display, an image sensor and the like, there is a need for producing a semiconductor device such as a thin film transistor on a transparent insulating substrate so that the driving circuit, which is to be externally mounted in prior art, is mounted on the same substrate as the liquid display, the image sensor, etc. Accordingly, various production techniques are being developed.
For example, when producing a thin film transistor (TFT) on a substrate, a production method including the step of implanting impurity ions to a semiconductor film which becomes a device formation region (active region) while using the gate electrode as a mask, thereby self-aligningly forming an n-type or p-type source/drain region, is useful because a channel length of the TFT to be formed can readily be shortened for higher levels of performance. Accordingly, its development is in progress.
In order to realize lower production cost, it is preferable to use an inexpensive glass plate as a substrate. For that purpose, it is necessary that the process temperature typically be about 600.degree. C. (preferably about 500.degree. C. or less) when heat resistivity of the glass substrate is under consideration. However, it is difficult to form a silicon semiconductor film of excellent quality at such a low temperature.
For example, if a polycrystalline silicon semiconductor film is to be formed at a low temperature of about 600.degree. C. or less, it is generally preferable that an amorphous silicon semiconductor film is first grown on a substrate. Then, the amorphous silicon semiconductor film is annealed for crystallization (solid-phase growth) so as to obtain the polycrystalline silicon semiconductor film. The reason is that the polycrystalline silicon semiconductor film having a large crystalline granular radius and excellent characteristics can be obtained.
However, in order to have the crystallization mentioned in the above process, it is generally necessary to perform annealing at about 600.degree. C for about 12 hours (preferably for about 24 hours). Heat treatment for longer hours is also required for a semiconductor device having higher field effect mobility or higher reliability.
However, longer hours of annealing results in a low throughput. Moreover, if annealing is performed at high annealing temperature for long hours, then the glass substrate may experience irreversible thermal contraction or warping. In order to resolve this situation, it is necessary to cope with two problems, one of which is to lower the annealing temperature and the other of which is to shorten the processing hours.
Described in Japanese Laid-Open Patent Publication No. 6-244103 as one of the solutions to the above-mentioned problem is a method where, by using a catalyst such as nickel (Ni) or the like, the crystallization of the amorphous silicon semiconductor film is performed at a temperature lower than the typical crystallization temperature. However, it is also described in the publication that the Ni atoms (catalyst) themselves are not desirable in the silicon film which is present as a semiconductor material. For this reason, in order to guarantee high quality of the crystalline silicon semiconductor film and high performance of the semiconductor device such as TFT which is formed of such a crystalline silicon semiconductor film, it is necessary to remove the Ni atoms (catalyst) which were added and used in the crystallization step.
Alternatively, in the case where such step of removing the catalyst element is not performed, it is necessary to control a concentration of catalyst element to be introduced to such a degree that the concentration is sufficient for prompting the crystallization of the amorphous silicon semiconductor film and, at the same time, the catalyst element which remains in the crystalline silicon semiconductor film does not have a negative effect on the characteristics of the crystalline silicon semiconductor film. However, the concentration of the catalyst element satisfying such requirements is typically very small. Accordingly, it is extremely difficult to realize the step of introducing the catalyst element while accurately controlling its concentration.
In addition to the above-described problem, in order to lower the production cost for a liquid crystal display or the like, it is strongly desired that the number of masks for photolithography needed during production be reduced, thereby simplifying the production steps. If the number of masks is reduced, then the production steps are simplified and a throughput and a production yield improve, thereby greatly reducing the production cost. However, the production method described in the conventional art requires more number of masks than generally needed, and therefore the above-mentioned objective, i.e., the simplification of the production steps, becomes difficult to achieve.